The oil and gas exploration industry makes use of a variety of instruments for measuring and controlling certain operations at depths of several thousand meters under the sea. For example an instrument which has been used for tracking pipeline pigs includes a radiation monitor and a data processor for detecting and recording the passage of a pig in its vicinity. The instrument display, showing the number of pigs detected and other information, can be read by a diver or remotely operated vehicle (ROV) deployed for the purpose. This known instrument is housed in a rugged and pressure-resistant metal housing, with the display visible through a glass lens. The lens is itself relatively thick and resistant to pressure down to about 3000 m. A problem encountered with these instruments is that the lenses are susceptible to breakage, particularly during an operation to retrieve the instrument from deep locations. The lens is therefore replaced between each deployment, thus increasing the cost of the instrument over its lifetime. A similar problem occurs with lights which are used in sub-sea conditions, e.g. portable lighting units used by divers or an ROV. It is an object of the invention to provide a display that overcomes this problem.